1. Field of the Invention
The present invention relates to a volume hologram photosensitive composition, a photosensitive medium for volume hologram recording using the composition, and a volume hologram using the same.
2. Background Art
Volume holograms are widely used in design, security and optical element applications, for example, because they can render an image three-dimensionally, have high diffraction efficiency and wavelength selectivity, and require advanced production techniques. Volume holograms are prepared by allowing object light and reference light, which are highly coherent and have the same wavelength, to interfere with each other and allowing the coherent light to enter a material for volume hologram recording to record, as interference fringes, three-dimensional information regarding the object in the inside of the material.
The interference fringes are recorded as refractive index modulation corresponding to the bright and dark parts of the coherent light. Excellent wavelength selectivity is one feature of the volume hologram, and only light of a particular angle or wavelength in a particular wavelength light source (incident light) can be diffracted for reproduction.
In recent years, in the production of volume holograms, attention has been made to a dry-type photosensitive composition for volume hologram recording that can eliminate the need to conduct wet development and can realize mass production. The dry-type photosensitive composition is composed mainly of a photoradically polymerizable monomer, a binder resin, a photoradical polymerization initiator, and a sensitizing dye and utilizes the difference in refractive index between a photopolymerizable monomer and a binder resin (for example, U.S. Pat. No. 2,664,234).
Specifically, upon interference exposure of a film of the photosensitive composition, photoradical polymerization is started in a high intensity light part, leading to diffusion migration of the photoradically polymerizable monomer. As a result, sparse and dense parts of the photoradically polymerizable monomer are provided depending upon the intensity of light according to an interference pattern, and this appears as a refractive index difference, whereby a hologram is developed. A large difference in refractive index between the photopolymerizable monomer and the binder resin and excellent storage stability after hologram formation are required of this dry-type photosensitive composition.
In order to satisfy the above requirements, for example, Japanese Patent Laid-Open No. 43634/1994 discloses a material for volume hologram recording, using a thermoplastic resin as a binder resin. The claimed advantage of this material is to enhance the diffusion migration of the photoradically polymerizable monomer and to realize a hologram having excellent brightness. Further, Japanese Patent Laid-Open No. 310932/2000 discloses a hologram recording material using a heat curing resin as a binder resin. The claimed advantage of this material is to realize a hologram having excellent film strength and heat resistance.
When the thermoplastic resin is used as the binder resin, the heat resistance and mechanical strength of the hologram are disadvantageously poor although the brightness of the hologram is improved. On the other hand, when the heat curing resin is used as the binder resin, maintaining a high level of refractive index modulation is not so easy and, thus, disadvantageously, a hologram having high brightness cannot be provided.